Page 92 - Materials Chemistry, Second Edition

8 Scope Definition 77

Input flows:
1. Materials
2. Energy
3. Resources
Output flows:
4. Products
5. Waste to treatment
6. Emissions.
Figure 8.1 shows a unit process of steel sheet rolling with an example of flows
for each of the six categories.
In practice, a unit process can represent a single process, e.g. the rolling of steel,
but it can also represent an entire facility that contains many different processes, e.g.
a slaughterhouse, if this offers the sufficient level of detail for the inventory mod-
elling. The latter type of unit process may be physically subdivided into two or
more new unit processes in a life cycle inventory model, see Sect. 8.5.4. Generally,
unit processes do not gain or lose mass over time and the sum of all input flows-
should therefore be equal to the sum of all output flows at the level of elements (e.g.
copper) and in aggregation.
Output flows belonging to the product or waste to treatment categories from one
unit process can act as input flows belonging to the categories materials and energy
for other unit processes and this is how unit processes are linked in a life cycle
inventory model. By comparison, resources and emission flows are not exchanged
between unit processes. They are referred to as elementary flows, and defined by
ILCD (using a slight modification of the ISO definition) as “single substance or
energy entering the system being studied that has been drawn from the ecosphere
without previous human transformation, or single substance or energy leaving the
system being studied that is released into the ecosphere without subsequent human
transformation”. The ecosphere can be understood as “the environment” and is
elaborated below. Note that a single substance should be seen as an ideal and that
some elementary flows in existing LCA practice are heterogeneous materials (such

Materials Product
Ex: Steel, unalloyed Steel sheet
Energy Unit process: Waste to treatment
Ex: Electricity Ex: Mineral oil
Steel sheet rolling
Resources Emissions
Ex: Water Ex: Particulates to air
Elementary flows

Fig. 8.1 The unit process of steel sheet rolling and examples of flows. The actual unit process
contains 86 flows [inspired by: ecoinvent v3 (Weidema et al. 2013)]